The present invention relates to pressure transducers, and more particularly to a method and pressure sensing system which corrects for errors of multiple pressure transducers each of which can be exposed to a unique environment of varying degrees of hostility.
In general, the use of piezoresistive, and in particular Wheatstone bridge, structures as pressure transducers is well known. Further, piezoresistive pressure sensors are used in many applications where they are exposed to fluctuating pressures to be measured at extreme temperatures. However, as is also known, as temperature fluctuates, so may the output of the transducer as the gage factor and resistivity of the sensor are both functions of temperature. Where these fluctuations are substantial enough to introduce a non-negligible error for the intended application, it is desirable to compensate for, or correct the induced error. A typical application where the temperature-induced error becomes non-negligible is aerospace applications, where a sensor can be subjected to extremely high pressures and extreme temperatures.
It is well known in the art the resistance of a Wheatstone bridge increases with increasing temperature and that the gage factor, which is the percentage change of resistance with increasing strain, decreases with increasing temperature. Thus, for a constant voltage applied to the bridge, the decrease in gage factor with increasing temperature leads to a decrease in bridge output at a given pressure. However, by putting a non-temperature varying resistor in series with the bridge, as the temperature of the bridge increases its resistance increases, and more of the supply voltage appears across the bridge. However, not only is the drop of the gage factor and thus the inherent change of the bridge output with constant bridge-excitation non-linear with increasing temperature, but the basic compensation technique of using a resistance divider is somewhat non-linear leading to a not-perfect compensation.
These problems can be overcome using an approach such as that illustrated in commonly assigned U.S. Pat. No. 4,192,005, entitled COMPENSATED PRESSURE TRANSDUCER EMPLOYING DIGITAL PROCESSING TECHNIQUES, the entire disclosure of which is hereby incorporated by reference. However, in the previous work a single digital correction circuit was required for each transducer and it was assumed each circuit would be attached to each transducer. Thus, the prior art approaches yield undesirably increased weight and cost for temperature compensated, or corrected, devices. It is desirable and an object of the present invention to provide a single error correcting system for use with multiple pressure sensors, and more particularly piezoresistive bridge pressure sensors, each of which can be exposed to unique environment. Each of the unique environments are at an associated temperature and have an associated pressure to measured by the sensor. In the preferred embodiment a degree of hostility varies between the unique environments. It is a further object of the present invention that the single error correcting system be functional with a number of devices which are not necessarily identical, and which are preferably adapted to measure different pressure ranges at different temperatures at different frequencies of measurement (i.e. some more often than others).
A method for measuring multiple pressures and a pressure sensing system for accomplishing the same. The pressure sensing system adapted for measuring a plurality of pressures includes: a plurality of pressure sensing assemblies each adapted to measure a corresponding pressure and be exposed to a respectively associated environment; a microcontroller; and, means for selectively coupling each of the plurality of pressure sensing bridge assemblies to the microcontroller in a predetermined sequence, wherein the means for selectively coupling are responsive to the microcontroller.
The method for measuring a plurality of pressures includes: exposing each of a plurality of pressure sensors to a corresponding plurality of environments each having a corresponding pressure to be measured; determining how frequently to measure each of the plurality of pressures; determining a sequence for utilizing the pressure sensors to measure the corresponding plurality of pressures, the sequence being dependent upon the determined frequency for each of the plurality of pressures; and, selectively utilizing each of the plurality of pressure sensors, according to the determined sequence, to measure the pressure to which it is exposed.